Question

An element has a half-life of 30 years. If 20 mg of this element decays over a period of 90 years, how many mg of this element would remain?

Answer 1

Answer:

2.5 mg

Explanation:

The half-life of a substance is the time required for one half of the substance to decay. In order words, it is the time it will take for a substance to decay into  half of its initial size.

For a 20 mg element with a half-life of 30 which decays over a period of 90 years;

The first 30 years, 10 mg would have decayed, remaining 10 mg

The second 30 years, the remaining 10 mg would have decayed to 5 mg

The third 30 years, 5mg would have decayed to 2.5 mg.

Hence, in 90 years, 20 mg of the element would remain only 2.5 mg.